The invention relates to a process for operating a fluidized bed combustor on coal basis in which comminuted coal is fed into a fluidized bed and burned with the bedding material. The invention furthermore relates to a fluidized bed combustor with a conveyor to feed the coal and with a nozzle bottom on which a fluidized bed consisting of bedding material is positioned during operation.
According to the present state of technology, coal is burned in fluidized bed combustion systems for steam and/or electricity generation (Siemens-Energietechnik 2 (1980), Issue 7, pp. 231-235). This results in two significant advantages. First, the combustion in the fluidized bed occurs at relatively low temperatures so that in comparison with known combustions using dry or even wet ash relatively little nitrogen oxide is formed Second, the combustion in the fluidized bed places relatively low requirements on the used coal, i.e. even a relatively roughly ground coal with high ash content may be burned without problems. "Ash" here means the incombustible substances contained in the coal, such as sand, clay, salts, metal compounds, etc.
When operating a fluidized bed combustor, coal is used which is ground to a grain size specific to the fluidized bed. It is fed to the fluidized bed, i.e. in most cases via the free space located above the fluidized bed. It is then mixed homogeneously with the bedding material. This bedding material consists both of sand and ash particles which were added to the fluidized bed with the coal, and of migrating coal particles.
A disadvantage here is that, in particular in the case of discontinuous ash removal, the fine-particle bedding material in the fluidized bed is continuously reduced because of flue-dust removal with the flue gas. The rough grain content increases steadily and settles during longer operation on the nozzle bottom of the fluidized bed. This again results in the obstruction of individual nozzles which feed the carrier air. As a consequence, locally differing zones with little or no air throughput at all are generated, as well as zones with increased air throughput. This significantly impairs the temperature homogeneity of the fluidized bed which is required for the removal of the generated heat, e.g. via a heat exchange system. In addition, temperature streaks form above the areas with increased air throughput in the free space of the fluidized bed combustor. These are narrow, long-stretched structures which are above the softening point of the ash. This results in slagging of the free space, e.g. in the form of nests. These slaggings may make it necessary to take the fluidized bed combustor out of operation. It is therefore important to balance the temperature distribution as evenly as possible over the entire fluidized, bed so that no slag deposits may form.